1. Field of the Invention
The present invention relates to a motor drive apparatus equipped with a dynamic braking circuit fault detection capability, and more particularly to a motor drive apparatus having a capability to detect faults in a dynamic braking circuit (such as a contact welding defect or an electrically inoperable contact, breakage of a resistor, disconnection of a connecting cable, etc.).
2. Description of the Related Art
FIG. 7 is a block diagram showing the configuration of a motor drive apparatus according to the prior art.
In the motor drive apparatus 101 shown in FIG. 7, a DC power supply 102 converts AC power, supplied from an AC power supply not shown, into DC power. The DC power from the DC power supply 102 is supplied to a synchronous motor 104 (hereinafter referred to as the motor) via a power transistor unit 103 comprising power transistors A to F. When the power to the motor 104 is cut off while the motor 104 is running, a dynamic braking circuit 105 actuates switches S1 and S2, i.e., relay contacts, so that the power is dissipated through resistors Ru, Rv, and Rw in the dynamic braking circuit 105.
A motor drive control circuit 110 outputs a dynamic braking circuit control signal SIG1, in response to which a fault detection circuit 111 generates a power transistor control signal SIG2 for controlling the driving/stopping of the motor 104, i.e., a power transistor ON/OFF signal, and supplies it to the power transistors A to F in the power transistor unit 103. A current detector 106 detects current flowing from the power transistor unit 103 to the motor 104.
The fault detection circuit 111 receives the dynamic braking circuit control signal SIG1 from the motor drive control circuit 110 and a contact state signal SIG70 from the dynamic braking circuit 105, i.e., an ON/OFF signal indicating the operation ON/OFF state of the motor 104 in accordance with the ON/OFF state of the switches S1 and S2, and detects from these signals a fault occurring in the dynamic braking circuit 105 (such as a contact welding defect or an electrically inoperable contact, breakage of a resistor, or disconnection of a connecting cable). To check the operation of the dynamic braking circuit 105 at the time of control of the motor 104, it is common to use the contact state signal SIG70 that indicates the contact state of the switches S1 and S2 as hardware provided in the dynamic braking circuit 105.
Methods that do not use such a contact state signal are also known; as one such method, there is disclosed, for example, in Patent Publication No. 1, a method that checks the operation/non-operation of the dynamic braking circuit by lowering the DC voltage when starting the motor operation.
[Patent Publication No. 1] Japanese Patent No. 3383965 (refer to [CLAIM 1] in Patent Claims, paragraphs [0003] to [0006] in Patent Specification, and FIGS. 1 and 2)
The method that requires the addition of a hardware contact signal involves the problem that the apparatus cost increases. On the other hand, the method of Patent Publication No. 1 that does not use such a contact signal requires the addition of a control circuit in order to perform control to obtain a low supply voltage necessary for fault detection, and this also involves the problem that the apparatus cost increases.